An optical modulator having an optical waveguide structure in which an optical waveguide is formed on an electro-optic substrate has been known as a mature technology. A Mach-Zehnder interferometer type optical modulator has a structure in which light waves propagating through different optical waveguides interfere with each other, and the optical intensity of light after the interference is determined by the difference in the amount of phase shifts between the light waves propagating through the optical waveguides (for example, refer to Patent Literature No. 1). By changing the difference in the amount of phase shifts, it is possible to change the output optical intensity of the optical modulator.
In the Mach-Zehnder interference type optical modulator disclosed in Patent Literature No. 1, the vicinity of an optical waveguide may be trenched to form the optical waveguide in a ridge shape. By forming the optical waveguide in a ridge shape, the effective overlap integral of the electric field generated by the electrical signal and propagating light wave in the optical waveguide can be improved. Accordingly, it is possible to reduce the driving voltage. In addition, by forming the optical waveguide in a ridge shape, the effective refractive index (microwave effective refractive index) of a high-frequency control signal (electrical signal) and the characteristic impedance are optimized. Accordingly, it is possible to improve the microwave characteristics.